1. Field of the Invention
The present invention relates to semiconductor devices for use in high-power high-frequency circuits provided with surface acoustic wave (SAW) devices, and in particular, to a semiconductor device for use in transmitting/receiving circuits for radio communication using portable units.
2. Description of the Related Art
In portable radio communication terminals, SAW devices such as SAW filters and SAW duplexers are used for the purpose of, for example, separation of a received signal, or suppression of an unnecessary signal. Japanese Unexamined Patent Application Publication No. 8-274575 discloses an attempt to achieve size reduction of a transmitting/receiving semiconductor device by mounting a SAW-device-formed chip on a circuit substrate having a high frequency circuit formed thereon. The circuit substrate used in the semiconductor device disclosed in the above Publication is described below.
FIG. 1 is a sectional view of a circuit substrate of the prior art in which a SAW device chip 3 is provided to a Si substrate 30 having a high frequency circuit formed thereon. Referring to FIG. 1, the circuit substrate has an insulating film 31 formed on the Si substrate 30, a passive element pattern 33 and a capacitor 35 which are formed on the insulating film 31. The passive element pattern 33 includes passive elements such as a thin film resistor, a thin film capacitor, and a thin film inductor, and are electrically connected to pads 32 formed on the insulating film 31.
The SAW device chip 3 is bonded to the pads 32 above the passive element pattern 33 so that interdigital transducers constituting a SAW pattern are directed downward, with bumps 34 provided therebetween.
In the circuit substrate, the SAW device chip 3, on which a SAW device is formed, is directly mounted on the Si substrate 30. Accordingly, it is not necessary to accommodate the SAW device chip 3 in a dedicated housing, so that the size of the transmitting/receiving semiconductor device can be reduced. Also, since the SAW device chip 3 is formed above the passive element pattern 33, which generates a small amount of heat, a change in the characteristics of the SAW device chip 3, which has large dependency on temperature, can be suppressed.
The above-described circuit substrate can be made of one of ceramic material, glass, sapphire, etc., instead of Si. Nevertheless, insulating substrates made of these materials have great differences in physical and chemical properties, for example, specific heat and surface roughness. Accordingly, chemical vapor deposition (CVD) conditions for forming passive elements on such the above insulating substrates are greatly different from CVD conditions for the semiconductor. Also processing conditions such as substrate etching are different from those for the semiconductor. Therefore, when the insulating substrates made of the above materials are used, it is difficult to use the semiconductor process to form passive elements.
Portable communication devices, for example, devices used as mobile communication terminals such as cellular phones, must handle high frequency waves of approximately 800 MHz to 1.9 GHz by using large power. Accordingly, it is required that high density such a large-output high-frequency circuit be integrated at a high density without deteriorating power efficiency.
It is an object of the present invention to provide a small semiconductor device that efficiently handles a high-power high-frequency signal by achieving size reduction using a SAW-device-formed chip directly provided to a circuit substrate and by realizing a high frequency circuit having superior impedance matching characteristics and low resistance loss.
To this end, according to the present invention, a semiconductor device is provided which includes a passive element chip including a passive element circuit including, as circuit elements, passive elements which are provided on a semi-insulating compound-semiconductor substrate in which one surface thereof is set to have a ground potential and in which the other surface thereof has neither transistor nor diode, and a surface-acoustic-wave-device chip on which a surface acoustic wave device is formed. The surface-acoustic-wave-device chip is provided on the passive element chip so that the surface acoustic wave devices are electrically connected to the passive element circuit. The passive element circuit includes a transmission line which is formed on the semi-insulating compound-semiconductor substrate and which functions in combination with the surface having the ground potential.
Preferably, the semi-insulating compound-semiconductor substrate is one of a GaAs substrate and an InP substrate.
The passive elements may include one of a resistive element, a capacitive element, and an inductive element.
The surface acoustic wave device may be one of a filter and a duplexer.
The passive element chip may be electrically connected to an active element chip including active elements.
The passive element chip and the active element chip may be accommodated in a single housing.
The substrate of the active element chip may be one of a GaAs substrate, an InP substrate, and a Si substrate.
The active elements may include one of a field effect transistor, a high-electron-mobility transistor, a Si bipolar transistor, a hetero junction bipolar transistor.
According to the present invention in which a transmission line provided on a substrate made of semi-insulating compound semiconductor, the wide transmission line having a high characteristic impedance is formed, and by using a semiconductor process, a passive element circuit that uses the transmission line to establish good impedance matching with a SAW device can be produced. Therefore, a semiconductor device that transmits or receives a high-power high-frequency signal can be realized and greatly contribute to improvements in the performance of radio communication devices.